The monitoring of ignition systems for gas turbine engines is of particular interest because such systems are of critical importance to the safe operation of aircraft incorporating these types of engines. By monitoring the performance of ignition systems in gas turbine engines, an indication that the system is malfunctioning can be obtained. By providing an indication of a malfunctioning ignition system, a measure of safety is obtained that can be of particular importance in ensuring the ignition system is capable of restarting an engine after a flameout has occurred, or to initiate a maintenance cycle prior to the next flight.
In monitoring ignition systems of gas turbine engines, the state of health of the igniter plug has in the past been the focus since the igniter plug is the component of the ignition system with the shortest average useful life. Failure or malfunctioning of other components of the ignition system, however, may occur and the typical monitoring system fails to identify failures or malfunctioning of these other components. Indeed, some monitoring devices may actually falsely indicate a properly operating ignition system when the system is in fact malfunctioning or failing, others may indicate a failure when none exists causing unnecessary maintenance.
An ignition sequence is typically initiated by a narrow high voltage pulse generated by an exciter circuit. For a successful ignition, the high voltage pulse is discharged at the igniter plug, thereby generating a spark. There have been attempts to analyze the voltage pulse from the exciter and the following voltage waveform generated by the spark in order to diagnose the health of an ignition system. In the past, however, such monitoring systems could only provide an indication of the health of the igniter plug and failed to monitor or diagnose the state of health of other components of the ignition system that may lead to failure of the igniter plug.
For example, U.S. Pat. No. 4,760,341 to Skerritt discloses a monitoring device that senses the electric field generated by a signal at the input to the igniter plug of an ignition system. The monitoring device receives the signal generated by the electric field and detects if the input signal to the plug is maintained longer than a predetermined time period and above a predetermined voltage level. If the input signal is maintained longer than the predetermined time period, the device indicates the plug is deteriorating. If the voltage of the input signal fails to reach the predetermined level, however, the monitor of the Skerritt patent also interprets this failure as a deteriorating plug when in fact the exciter may be degraded and the igniter is functioning properly.
In addition to measuring the width of the high voltage pulse, the monitoring device in the Skerritt patent also measures the energy discharged through the plug during the spark event. If the total energy delivered in the spark event signal is satisfactory and the signal to the plug is not too long, the monitoring device provides a pulse output, indicating that the igniter plug is operating properly.
Although measurement of the total energy delivered to the plug in response to the high energy pulse is a useful complement to the measurement of pulse duration, the two measurements fail to provide the user with anything other than a general indication that the ignition system is malfunctioning. More specifically, the measurements of the Skerritt monitoring device do not distinguish between a failing device for discharging the high energy pulse (i.e., the igniter plug) and a failing device for generating the high energy pulse (i.e., the exciter).